1. Field of the Invention
The present invention relates to a projection optical system for use in a projection type image display apparatus that enlarges and projects an image at a finite distance.
2. Description of the Related Art
Among the projection optical systems such as the one described above, a retrofocus type lens having a lens unit with negative optical power arranged before the other lenses is characterized by covering a relatively large angle of view and by easily ensuring a long back focus as compared with the focal length.
On the other hand, since the lens system itself is asymmetric, it is difficult to correct distortion, astigmatism, or comma aberration. In particular, the optical system must be made long enough to allow distortion and astigmatism to be appropriately corrected.
Some of such retrofocus type lenses which are used for film cameras employ a rear focus type that moves a rear lens unit in the lens system for focusing. One of such retrofocus type lenses is proposed in, for example, Japanese Patent Laid-Open No. 7-35974 or the like.
In general, the rear focus type requires a focusing lens unit to be moved by only a small amount as compared with an entire focus method, which requires the entire lens system to be moved. The focusing lens unit of the rear focus type is small and light as compared with the entire focus method. Consequently, the rear focus type enables focusing to be achieved with only small driving force and is thus particularly suitable for cameras having an automatic focus detecting device for a film camera, or the like. Furthermore, the length of the lens remains constant in spite of focusing. Thus, the rear focus type has advantages such as the capability of allowing an image-taking apparatus to be held easily to prevent camera shake.
On the other hand, in connection with applications of a projection type image display apparatus using an image forming element such as a liquid crystal panel, it is desirable to develop, in order to accommodate various projection conditions, a wide-angle projection lens that can project a large image at a short projection distance so that the image appears bright from the center to corners of the image.
Further, a so-called telecentric system, which has an apparent pupil position at a point of infinity, is desired in order to eliminate the effects of light distribution characteristics of a liquid crystal (in the case of a projection type image display apparatus using a liquid crystal display panel) and the effects of the dependence of a color combining dichroic film on angle during combination of lights of a plurality of colors and to sufficiently match with the illuminating system to ensure a sufficient luminance in the peripheral portion of a projected image.
However, the conventional rear focus lens has a short distance between a reducing-side conjugate plane and a pupil, resulting in inadequate alignment of the pupil with the illuminating system. This reduces the quantity of light in the peripheral portion of the projected image and makes luminance nonuniform. Therefore, this rear focus lens is insufficient as a projection optical system for a projection type image display apparatus.
Additionally, the optical power of the first negative lens unit must be increased in order to realize a compact wide-angle lens while ensuring a specified back focus (a reduction in projection distance). In this case, the asymmetry of the optical power arrangement increases to enhance the level of outgoing comma aberration during short-distance focusing. The level of astigmatism also increases, thereby significantly degrading optical performance.
Further, another focusing method is a so-called floating type that carries out focusing by moving the first lens unit with negative optical power and the second lens unit with positive optical power at different speeds. This method reduces variations in various aberrations during focusing. However, this method involves a complicated body tube structure and thus an excessively large lens diameter. Therefore, this method tends to increase the size of the body tube as well as costs.
It is an object of the present invention to provide a projection optical system that exhibits high optical performance by appropriately correcting variations in aberrations during focusing irrespective of the distance at which an object is located, i.e. whether the object is located at a point of infinity or at a short distance, while maintaining the advantages of the rear focus retrofocus type.
To attain this object, a projection optical system according to the present invention, which enlarges and projects an image comprises in order from the enlarging side:
a first lens unit having negative optical power; and
a second lens unit having positive optical power, said second lens unit moves in a direction of an optical axis for focusing.
The optical power is the inverse of a focal length.
Further, the following condition is satisfied:
0xe2x89xa6|f/t1| less than 0.2
where reference character t1 denotes a distance from the reducing-side conjugate plane to a pupil, and reference character f denotes a focal length of the entire system.
A detailed configuration of the projection optical system, projection type image display apparatus, and image projection system of the invention, the above and other objects and features of the invention will be apparent from the embodiments, described below.